Dendritic zinc electrodes



Jan. 1, 1963 M. B. CLARK ETAL DENDRITIC zmc ELECTRODES 3 Sheets-Sheet 1Filed June 25, 1959 INVENTORS MILTON B. CLARK By GEORZIE R.DRENGLERATTORNEY a 1963 M. B. CLARK ETA. 3,071,638

DENDRITIC ZINC ELECTROiDES Filed June 25, 1959 3 Sheets Sheet 2 mmvroxsMILTON B. CLARK By GEORGE DRENGLER A T TORNEY Jan. 1, 1963 M. B. CLARKETAL 3,071,638

DENDRITIC ZINC ELECTRODES Filed June 25, 1959 5 Sheets-Sheet 3 INVENTORSMILTON B.CLARK By GEORGE R.DRENGLER jLQM A T TOR /V15) United States3,071,638 Patented Jan. 1, 1963 This invention relates to thefabrication of high efficiency, high current density zinc electrodesfrom electrochemically deposited zinc powders.

To obtain satisfactory high current density performance from a zincanode in an alkaline medium, the available area of the zinc must be aslarge as possible. This measure results in more efficient zincutilization, and makes possible high current density operation for thezinc oxide film formed during discharge is spread over a larger area,and hence Will require a longer period of time to attain a suflicientthickness to result in passivation. Various methods of increasing thesurface area of zinc electrodes have been suggested previously andinclude:

(1) Spraying molten zinc onto a suitable support.

(2) Using powdered zinc in a gelled electrolyte, either in bulk orpasted on a grid.

(3) Reducing electrolytically zinc oxide which has been pasted on agrid.

(4) Using pasted or pressed electrodes made from zinc powder or dust.

Commercial zinc powder used in making zinc anodes is obtained as acondensate in the distillation of zinc from roasted zinc sulfide oreblends, or by atomizing molten zinc with a blast of air. Either of thesemethods results in a zinc powder which contains as much as 10 percent ofzinc oxide. Zinc powders can be made by electro-deposition, althoughthere appears to be no commercial source for this type because of thepyrophoric property of the material.

The main object of the present invention is to provide a high efficiencyzinc anode molded from a specially prepared electrolytic zinc powder.

An equally important object of the present invention is to provide meansfor preventing high surface zinc powders from becoming pyrophoric priorto their use in achieving the main object of the invention.

The present invention whereby the foregoing objects are attainedcomprises compressing dried amalgamated electrolytically deposited zincsponge into an electrode. A screen or expanded metal grid may bemolded-in for added strength but is not essential.

In the drawings:

FIG. 1 is a micrograph showing electrolytically deposited zinc particlesunder 333 fold magnification;

FIG. 2 is an electron micrograph showing electrolyti: cally depositedzinc particles under 11,500 fold magnification;

FIG. 3 is an electron micrograph magnified 11,500 and showing commercialreagent grade zinc particles;

FIG. 4 is a front elevational view of a support grid for the anode ofthe invention;

FIG. 5 is a front elevational view of an anode in accord with theinvention; and

FIG. 6 is a perspective view of an assembly for making duplexelectrodes.

During the course of experimentation leading to the present invention,very finely divided sponge zinc was prepared electrochemically, whichprobably would be an excellent anode material, except that after drying,it could not be exposed to air without spontaneous combustion.Unexpectedly it was found, however, that amalgamation of this spongezinc before drying yielded a stable material which did not ignite afterdrying. The use of this amalgamation step allows one to make a veryfinely divided zinc powder, which can be dried safely and fabricatedinto an excellent electrode for high current density application byforming it into a plate under pressure.

Illustrative of the practice of this invention, sponge zinc wasdeposited from a solution having the following composition:

Grams/liter NaOH 200 Zn (added as ZnO) 10 A fiat zinc sheet was used asa cathode, and an amalgamated zinc plate as anode. The current densityused was 1 amp/sq. in., and the process was carried out at roomtemperature. After one hour electro-deposition time, the cathode wasremoved. The deposited sponge zinc was scraped off and rinsed with waterto remove excess alkali. It was then transferred to a fiashk containingdistilled Water slightly acidified with HCl (5 drops/ cc.), in which wasdissolved suflicient mercuric chloride to give 24 percent by weight ofmetallic mercury with respect to sponge zinc. The flask was shaken for asufficient period of time to insure complete amalgamation, after whichthe amalgamated zinc sponge was filtered and dried in a vacuumdesiccator at room temperature.

Pressed electrodes were prepared as follows:

Zinc sponge powder as prepared above was spread into a mold cavity andpressed. Various fill depths tried successfully ranged from 0.030 inchto 0.230 inch, with resulting pressed plates of about 0.015 to 0.100inch thickness.

The powder was molded onto a copper screen at a pressure of 0.5 ton/m Atthe molding pressure the sponge zinc of the electrode has an apparentdensity of about 1.85 g./cc. equivalent to 74 percent porosity.

Duplex electrodes were also molded in which the anode was made from theabove described zinc sponge. In this case the sponge was pressed throughan expanded metal grid and against a thin metal shim.

Zinc sponge, when prepared in the manner of the invention, is dendriticin nature (FIGS. 1 and 2), and has an extremely large surface area. Onthe other hand, commercial zinc dust (flue dust) is comprised of minutespherical particles (FIG. 3). The latter material cannot be pressed intoan electrode of satisfactory physical properties without application ofexcessive pressure which compresses the material and reduces itsefiective surface area to such an extent that it becomes unusable as ahigh current density anode.

A comparison of the zinc utilization efliciency performance ofelectrodes prepared in accord with the invention and electrodes madeotherwise appears in Table I below. The percent utilization efficiencyis 100 times the ratio of the experimentally determined coulometercapacity of the electrode to the theoretical capacity calculated fromthe total zinc present. The electrode representative of the inventionconsisted of electrolytically deposited zinc sponge molded onto a copperscreen under a pressure of 0.5 ton per inch square. In each instance,tests were run against a nickel cathode in 7.35 M potassium hydroxide. Acurrent interrupting device was used which permitted measurement of theanode potential versus a reference electrode consisting ofmercurymercuric oxide, without the inclusion of the voltage (IR) dropthrough the electrolyte. These efiiciency tests were terminated at a 1.0v. cut-off. It will be seen that the percent utilization efiiciency ofthe present anode (sample I) exceeds considerably that of the othertypes (samples 2, 3, 4 and 5).

3 TABLE I Electrochemical Performance of Various Types of Zinc AnodesAMBIENT ROOM TEIWIERATURE (Nondendritic zinc) presumably formedelectrolytically from ZnO pasted plate.

Zinc particles made in accord with the present invention also may beused in making electrodes of types other than those described. Suchpowders as herein described may be used with plastic binders to formself-supporting anodes in accord with the method of US. Patent 2,828,-351 to H. R. Rade..

What is claimed is:

1. In an alkaline galvanic cell, the improvement which consists of ananode characterized by a high surface area, said anode comprisingcompressed, finely divided, electrochemically deposited, amalgamated,dendritic zinc particles.

2. In an alkaline galvanic cell the improvement which consists of ananode characterized by a high surface area, said anode comprising ametallic grid embedded in finely divided, electrochemically deposited,amalgamated dendritic zinc particles.

3. The improved alkaline galvanic cell of claim 2 wherein said grid isof copper.

4. A process for making high current density dendritic zinc electrodescomprising electro-depositing dendritic sponge zinc from an alkalinesolution containing a hydroxide of an alkali metal and zinc, removingthe electro-deposited zinc, removing excess alkali from said zinc,agitating said zinc with an acidified amalgamated solution, filteringthe thus amalgamated zinc drying the thusobtained amalgamated dendriticzinc and forming the same to desired electrode shape.

5. The process of claim 4 wherein the dried amalgamated dendritic zincis molded onto a metallic screen to form a reinforced electrode.

References Cited in the file of this patent UNITED STATES PATENTS GreatBritain July 16, 1931 UNITED STATES PATENT OFFICE Certificate ofCorrection J a-nua/ry 1, 1963 Patent No. 3,071,638

Milton B. Clark et al.

the above numbered patent requiring hould read as corrected below.

S m n m we a hr 2% 6 mL m aw m mt a m m Figs. 4, 5, and 6 as shownbelow, should be contaming rted as part of the Letters Patent In thedrawings, Sheet 4 inse M. B. CLARK ETAL DENISRITIC ZINC ELECTRODES Jan.1, 1963 4 Sheets-Sheet 4 F iled June '25, 1959 VlsNTbRs GEORGE R.DRENGLER Arrok/wsr in the heading to the drawings, Sheets 1 t0 3, line3, for 3 Sheets, each occurrence, read-4- Sheets; column 2, line 19, forflashk reedflask.

Signed and sealed this 23rd day of July 1963.

[SEAL] Attest:

ERNEST W. SWIDER, DAVID L. LADD,

Attesting Ofiicer. Gammz'ssz'ouer of Patents.

1. IN AN ALKALINE GALVANIC CELL, THE IMPROVEMENT WHICH CONSISTS OF ANANODE CHARACTERIZED BY A HIGH SURFACE AREA, SAID ANODE COMPRISINGCOMPRESSED, FINELY DIVIDED, ELECTROCHEMICALLY DEPOSITED, AMALGAMATED,DENDRITIC ZINC PARTICLES.